Excess-voltage protective structure



' y 1935. R, R. PITTMAN EXCESS VOLTAGE PROTECTIVE STRUCTURE Filed Jan.11, 1954 3 Sheets-Sheet 2 INVENTOR VMG W y 1935- R. R. PITTMAN 2,002,042

EXCESS VOLTAGE PROTECTIVE STRUCTURE Filed Jan. 11, 1934 5 Sheets-Sheet 3INVENTOR @w PM Patented May 21, 1935 PATENT OFFICE excess-venuesraorsc'nva srauoruas Ralph a. Pittman, Pine mun, Ark.

Application January 11, 1934, Serial No. 708,170

13Clalms.

This invention relates generally to excess-voltage protective devicesfor use with high voltage alternating current electrical systems, andhas for its principal object the provision of means for positivelypreventing an excessive superimposed voltage rise on a conductor orconnected apparatus, such as might be caused by lightning.

It is a particular object to provide a novel arrangement for the purposeof effecting the cascade flashover of a series arrangement of spark gapsformed by a plurality of normally insulated conducting electrodessupported in spaced relation.

A further object is the provision of a novel means for reducing the timelag of flashover of such a series arrangement.

A further object is the provision in an are extinguishing deviceembodying one of the spark gaps of such a series arrangement, of meansfor 20 reducing the rate of recovery voltage rise, thereby facilitatingthe extinguishment of the arc.

A further object is to provide means for reducing the voltage stressnormally imposed on the insulating members of the arc extinguishingdevice to prevent thermal damage to such members from creepage current.

A yet further object is the provision in an arc extinguishing device ofthe expulsion type, of a novel discharge path a providing a low initialbreakdown voltage, andv an arcpath which may subsequently be taken by anarc following sparkover which varies both in length and cross-sectionalarea, the latter quantities being so related under all conditions as toeffect are extinguishment.

With the above and other objects in view which will appear from thedescription, my invention resides in a novel electrostatic field controlarrangement, in combination with a novel type of arc extinguishingdevice in which is embodied certain members which assist in arcextinguishment, and at the same time effect the desired electrostaticfield control, thus making possible mum of parts, and greatlysimplifying the structure.

In the drawings:

Figure 1 illustrates an assembly of one embodiment of the structure,shown in perspective.

Figure 2 is a diagram of a curve showing a volttime characteristic of mystructure, in comparison with similar characteristics of certainwell-known types ofprotective gaps.

Figure 3 is an elevation of the arc ext nguishing 65 device, shown insection.

the attainment of the desired results with a mini- Figure 4 is anotherview of the arc extinguishing device, as seen from the plane indicatedby the line 4-4 in Figure 3.

Figure 5 is an additional view of the are extinguishing device, as seenfrom the plane indi- 5 cated by the line ll in Figure 3.

, Figure 6 is a fragmentary plan view of the mounting base of the arcextinguishing device, illustrating the venting means.

Figure 7 is a perspective of the cylindroid 10 forming the upperelectrode of the arc extinguishing device.

Figure 8 is an elevational view of another embodiment iii the invention,shown partly in section.

Figure 9 is an elevational view of yet another embodiment of theinvention, also shown partly in section.

Referring now in detail to the embodiment of the invention illustratedin Figure l, a portion of ausual type of steel substation supportingframework, comprising the steel members 35, 36, and 31 is designated inits entirety by the numeral II. In accordance with usual practice, thisframework is maintained at substantially ground potential, and is hereshown connected to the ground 50. An insulating member, or bus support,which may be an assembly of several insulating units, is designated by il, and extends in a downwardly direction from the steel member 31, beingattached thereto at its upper end in any suitable manner, as by the capscrews 38. An upper conducting member i1, here shown as a metal ring isrigidly supported in coaxial relation with the insulating member ll, bymeans of a pair of outwardly and downwardly extending metal arms 28,having their respective ends attached to the ring I! by the bolts IO,and to the lower end of the insulating member II by the bolts ll. One ofthe bolts ll also provides a means for connecting a line conductor II tothe lower end of the insulating member II.

A conducting electrode 2|, here shown as a metal rod, is also supportedin any suitable manner at the lower end of the insulating memher H, andextends downwardly therefrom coaxially with the ring I1 and theinsulating member ll.

From the above description it will be apparent that both the ring I! andthe electrode 20 60 are at the same potential as the conductor l3. Forreasons which will later appear, the diameter of the ring II should belarge in comparison to the diameter of the electrode 20.

A second conducting member, as the metal ring l8, of substantially thesame size as the ring I1, is rigidly supported in coaxial spacedrelation therewith by means of the metal arm 22, the latter having itsrespective ends attached to the ring l8 by the bolt 23, and to the steelframework member 36 by the bolts 24. Since the supporting framework ismaintained at substantially ground potential, it will'be apparent thatthe voltage of the conductor with respect to ground potentialisdefinitely limited I to the value determined by the spacing of the ringsl1 and It.

An arc extinguishing device, for convenience designated in its entiretyin Figure i by-the numeral l2, and shown more in detail in Figures 3, 4,5, 6 and 7, is supported coaxially with the electrode 20 by means of theflange 32, which may be attached by means of cooperating threads to thelower end of the device l2, and the bolts 34 cooperating with the flange33 and thexcon-q ducting member,

- The arc extinguishing'device I 2, as shown in Fig. 3. andlater-described more in detail, in-

is preferably determinedby 'the' insulation cludes the upper electrode29 and the lower electrade 32, which are normally insulated from eachother, and means for extinguishing an arc-which may occur therebetween.At the upper end of the arcextinguishingdevice l2, 'andelectrically ingmetal arms '26, at the respective ends of which, and in coaxial spaced.relation with the rings l1 and I8, and the'arc extinguishing device I2,is supported, by means of the bolts 21,

V a third conducting member or metal ring 19.

The latter is substantially the same size as the rings l1 'and 18. Anelectrode 2|, here shown as a metal rod, is carried in any suitablemanner at the upper end of the arc extinguishing device I2, and extendscoaxially inanupward direction toward but in spaced relation with theelectrode 20, to form a spark gap therebetween.

From the above description, it will be apparent conductorto ground; thefirst between the line conducting member or ring I1 and the groundconducting member or ring. 18, and the second from the electrode 20 tothe electrode 2|, and

thence through the arc extinguishing device I 2 to the grounded member35.

Assuming now that a highvoltage with respect to ground potential isimpressed upon the conductor l3, it will be apparent that thedistribution of the resultant electrostatic field between the rings l1and 18 will retardthe formation of streamersfrornthe electrodes and 2|,thus reducing the time lag of flashover therebetween.

It will also be seen that all of the flux passing.

from the electrode 20V to the electrode 2| is further divided betweenthe rings l8 and I9, and

from line to ground, one of which is embodied in the arc extinguishingdevice l2. As the voltage division between the two above-mentionedseries gaps is dependent upon the relative flux division, or therelative shunt capacitance of the gaps, it will beseen that the voltagerequired to sparkover the series arrangement may bemade substantiallyequal to that required to sparkover the longer of the two gaps. Thespacing between the rings 11 and I8 must therefore be somewhat greaterthan-the electrode spacing of the longer of the two series gaps, but maybe made substantially less than the sum of the spacings of theelectrodes forming the series arrangement.

It will be seen'that the ring l'I cooperates with the ring II to providea capacitance in shunt" strength of the apparatus to be protected, thusproviding a definite coordination gap having a desirable volt-timecharacteristic. The spacing of the electrodes forming the seriesarrangement to ground may then be adjusted to prevent 'sparkover of thecoordination gap, since the series arrangement embodies the arcextinguishing device for preventing a circuit outage which would occurshould the coordination gap sparkover.

better illustrated by Figure 2, in which the line a may represent thev'olt-time breakdown characteristic or an electrical apparatus to beprotected. The line b represents the characteristic of theabove-mentioned coordination gap formed by vthe'spaced rings IT and II,the slope of the line being determined by the physical dimensions'of'the rings employed, and it is thereby adjusted to lie below andsubstantially parallel to, the characteristicof the apparatus 'tobeprotected. Theline c of the protective structure, due to the .novelarrangement of .the conducting members, may be adjusted to flashover atsubstantially lessvoltage than either the apparatus or the coordinationgap for those time intervals characteristic of the rate of voltageapplication due to natural lightning; that is, the characteristic of theprotective structure in which is em-, bodied the arc extinguishingdevice lies.below and substantially parallel to that of the apparatusand coordination gap to be protected.

The lines d and-e of Figure2 represent respectively the characteristicsof ring-ring and rod-rod gaps which have anelectrode separation equal tothe ,sum .of the electrode spacings of series gaps embodied in theinvention. It is apparent that neither of these gaps ofiers desir- Thevolt-time characteristic of the invention is able protection to theconnected apparatus, not. I

only because the voltage required to cause-sparks over is much higherthan that of the protective such gaps are not inherentlyself-extinguishing. The are extinguishing device v illustrated inFigures 3, 4, 5, 6, and '7 wil now'be described indetail. a tubularhousing a, of suitable insulating material, is supported with itsprincipal axis normal to the metal mountlng'member 2i, h

and is provided at its lower end'with the metal flange 33. The bolts 34-extend through the flange 33 and the mounting member "to provide 5arrangement herein disclosed, but also because II. .by means ofcooperating threads on the r the top at the midpoint of the cap II andcoaxial with the housing Ill, together withfcooperating threads on thelower end of the spark gap elec- ,trode 2| provides a convenient meansfor rigidly mounting the latter on the upper end of the device. Anadditional pair of tapped holes 50 in' the cap it, located laterally ofthe electrode II, are provided for receiving the cap screws 25, shown inFig. 1. Eccentrically disposed within the housing 30, and slidablysupported with its base resting on the member 35, is the hollow tubularinsulating element 3 I, having its upper end completely closed by anintegral portion thereof, and open at its lower end. The inner surfaceof the element II is provided with a conducting coating 32, which ispreferably of arelatively high resistance as compared to an equivalentsection of copper. The coating may be in the form of a conductinggraphite or metallic paint, or anyconvenient ma- .terial having thedesired resistance characteristic. In addition to covering the entireinner surface of the element ii, the conducting coating extends over thelower end of the wall of the element 3|, to provide a lower electrodefor the device. The coating 32, being in contact with the supportingmember 36, is thereby maintained at substantially ground potential.

A hollow metal cylindroid 29, better shown in Figure 7, closed at thetop and open at the bottom, is closely fitted over the top of theelement II, in such a manner that it extends downwardly from the top fora portion of the length of the element SI and closelyengages the top andouter.

surface thereof. The outer surface of the cylindroid 2! is aligned withthe outer surface of the element I I, so that a continuous smoothexternal surface is provided on the assembly comprising these parts. Thetop of the cylindroid is held in engagement with the upper end of theelement 8! by means ofthe resilient conducting member II, the latterbeing positioned between the cap ll and'the cylindroid 29. The resilientconducting member 44 also serves toeleetricall'y connect these parts, tothe end that the cylindroid 28 is maintained at substantially the samepotential as the cap II and theelectrode 2i The cylindroid 29, incooperation with the conducting coating 32, provide respectively theupper and lower electrodes of the arc extinguishing device, and are sospaced longitudinally of thehousing 30 that the weakest electrical pathis internally of the housing, and along the inner wall thereof. I

, The wall of the" housing 30 is provided with a pair ofopenings, intoeach of which is fitted the hollow metal cylinders 4 I, a flange beingprovided adjacent one end of each of the cylinders for en? gaging theinner surface of the housing It. The

respective members. The coil springs 42, each having one end thereofengaging the caps 48, and the other ends engaging the slidable membersll, are housed within the cylinders II, and the slidable members 40engage the outer surface of the element 3| to resiliently urge it in alateral direction so that it engages in abutting relationthe innersurface of the housing 30 along a vertical line passing through thelowermost point of the cylindroid 28. Q

From the above description it will be seen that the shortest paththrough the device, and there-- fore the one of lowest dielectricstrength, will be along a line extending vertically from thelowermost-point of the cylindroid or upper electrode 20 to theconducting coating or lower electrode 32, and in a space between theinner surface of the housing and the outer surface of the element Ii. Itwill amo be seen that the pressure resulting from an are following thispath will be the highest per'unlt length of path, since this is the pathof maximum arc restriction. The pres sure incident to an are over thispath urges the member ii in a lateral direction, this motion beingopposed by the springs 62. Following such a movement, the arc may followany path from the upper electrode 29 to the lower electrode II, but, dueto the novel arrangement of the electrodes. as the restriction of theare becomes les as it moves around the periphery of the element 3 I, thelength of the arc becomes correspondingly greater. The device is in thismanner arranged to extinguish the arc regardless of the path takenbetween the electrodes. Obviously the advantage of the arrangement liesin the fact that the device is adapted to sparkover at a very much lowerinitial voltage than would be possible in a device having an arc path-ofconstant arc length and constant cross-sectional area of arc path, thisresult being accomplished without sacrifice of arcextinguishingqualities. In addition, the impact resulting from the operation of thedevice, due to the resilient mounting of the element II, is much lessthan that attending the operation of a rigid structure of the samearc-extinguishing effectiveness.

The housing I0 and the element 3| are preferably constructed of somematerial which will evolve gas in the presence of an arc, as hard fiber,since the device depends for its arc-extinguishing qualities upon thepressure resulting from an are within the device. The vents 38 in themounting member 35, as shown in Figure 6, are provided to relieve thestress due to such pressure, and the circumferential reinforcing bands39, located between the ends of the housing Iii, are desirable forproviding additional wall strength to enable the device to withstand theheavy pressures attending the interruption of large currents.

. The hard fiber, mentioned specifically above as a suitable materialfor use in constructing the housing 30 and the element II, is, in commonwith numerous other insulating materials of organic base,quite'susceptible to damage from creepage currents, particularly ifsubiected to high voltages for long periods of time. To prevent suchdamage, the structure embodies means for normally impressingsubstantially all the voltage-between the electrodes 2! and II, and inaddition provides means for preventing the concentration within thehousing of any flux entering the arc extinguishing device.

To further increase the arc-extinguishing qualof the element 3I"interposed therebetween, forms a capacitance in shunt relation to thevoltage tending to maintain the are within the device.

In an alternating current are, the current passes through zero twiceeach cycle, and the recovery voltage tending to maintain the arcincreases as a transient voltage at a rate determined by the constantsof the circuit of which the arc is a part. Obviously any capacity inshunt relation to the arc must be charged before the transient recoveryvoltage tending to sustain the arc'can increase.

A small capacitance in shuntcircuit relation with V respective seriesgaps from electrode 20 to the grounded member 35 through the arcextinguishthe arc has therefore considerable effect in reducing the rateof recovery voltage, and any suchreduction assists inarc-extinguishment.

desirable, however, that such a capacitance be so arranged and of suchdesign that in discharging it will not set up voltage oscillations ofsuch ma nitude as to defeat its, purpose. For this reason, the coating32 is preferably of a sufficiently high resistance to provide therequired damping effect to suppress such oscillations. I

It will be seen from the above description that the importantconsiderationsin applying the de-,

vice for operation on a circuit having a given normal voltage are (1)the weakest electrical path must-be internally of the housing 30, and inthe space between the housing and the element 3|,

and (2) the length of any path between the upper.

electrode 29 and the lower electrode 32 must be suflicient that, underthe conditions of operation, the normal circuit voltage will beincapable of sustaining an are over any path within the housing; 1

Additional embodiments of the invention here-, in described areillustrated inFiguresB and 9, in each of which has been incorporated theabove described advantages over prior art structures.

Referring in detail to Figure 8, it will be seen that the structure hereillustrated is similar in many respects to that shown in Figure l, theessential difference being the substitution of a dielectric materialhaving a greater dielectric permittivity than air for accomplishing afurther dstribution of the flux passing between the electrodes 20 and2|. p

The ground ring [8 in the embodiment shown by Figure 8 is supported by aplurality of pairs of insulating units. Each unit includes the porcelainbody 45 having rigidly secured in insulated relation the conductingmetal cap 55 and the conducting base member 46 at the respective endsthereof. The bolts 48, extending through the base member 46 and theframework member 35, rigidly secure the lower unit of each pair oneither side-of the arc extinquishing device l2. The clip angles 50,togetherwith the cap screws 5| cooperating with the caps 55 of the upperunits, and the bolts 52 cooperating with the ring I8, are provided forrigidly attaching the latter at the top of each pair of units. The ringl8 in this embodiment is maintained at ground potential by theconducting member 22, the respective ends of which are attached to theframework l by the bolts 24, and to the clip angle 50 by the bolt Thecap screws 41 extend through the base members 46 of each upper unit, andinto the caps 55 of each lower unit, rigidly connecting the two units ofeach pair together. One end of each of the conducting members 49 isfixed to the base member 46 of each upper unit, and the cap 55 of eachlower unit by means of the cap screws 41,

and the other end is attached to the cap l5 of the It is' areextinguishing device 12 by the cap screws-25.

It will now be seen that any flux passing from the electrode to theelectrode 2| is divided, in accordance with the respectivepermittivities of the various multiple paths, through the dielectric ofthe four insulating units in parallel, and in addition, as in each ofthe other embodiments herein described, through the dielectric. of. theelement 3| between the cylindroid' 29 and the conducting coating 32. Asaresult, the flux longitudinally of the arc path of the arc extinguishingdevice I2 is much less than that passing between the air-spacedelectrodes and 2!, thus providing the unbalance of voltage desiredacross the above described, provides the means for preventing thatfiuxpassing between the electrodes 20 and 2! from concentrating betweenthe upper and lower electrodes of the arcextinguishing device I2. Inthis structure the ground ring 18 is supported by means of a pairofoutwardlyand,

upwardly extending conducting arms 54, the lower ends of which areattached to the flange 33 by the bolts 34, and the upper ends to thering.

i8 by means of the bolts 53. In this arrangement, it will be seen thatthe ground ring I8 is so positioned in the electrostatic field of thestructure as to intercept substantially all- .of the flux which wouldotherwise pass from the upper ring I 1 to the cap 15 of. the arcextinguishing device, thereby efiectively preventing any flux enteringthe device i2 except that passing between the electrodes 20 and 2|. V

From the several embodiments of my invention herein described, otherequivalent arrangements will be readily apparent, and it is to beunderstood that the invention herein disclosed is to be limited only asis necessitated by the prior art and the appended claims. I

What I claim as new, and desire to secure by Letters Patent, is:

1. An excess-voltage protective arrangement adapted to be interposedbetween a conductor and ground, comprising a pair of conducting membersrigidly supported in spaced-apart relation, one of said members beingelectrically connected to said conductor and the other of saidv bersassociated with said electrodes for causing cascade flashover of saidspark gaps.

2.. An excess-voltage protective arrangement adapted to be interposedbetween a conductor and ground, comprising a pair of conductingmembers'rigidly supported inspaced-apart relation at a definite distanceone from the other, one of said members being electrically connectedtosaid conductor and the other of said members being electricallyconnected to ground, said pair of members forming a spark gap adaptedto'limit to a predetermined value the potential difference between saidconductor and ground, and

9,009,049 means for preventing fiashover between said members. saidmeans including a plurality of normally insulated conducting electrodesrigid- Ly supportedin spaced relation to provide a plurality oisparkgapsin series circuit relation be- ;tween said conductor and ground andcapacity .meansincluding said conducting members asso- .-ciated withsaid-electrodes for causing cascade fiashover of said spark gaps,the-saiddefinite distance being less than the sum oi'the electrodespacings oi said plurality of spark gaps. .3. Inan. excess-voltageprotective structure summit to a voltage from a conductor to ground.insulating means supporting a plurality of noroi said 'gapssubstantiallyall of said voltage on said first spark gap.

4. 1n excess-voltage protective structure, means supporting a pluralityof normally insulated conducting electrodes providing a plurality oispark gaps in series relation from conductor to ground, a hollow tubularinsulator having gas evolving wall material therein housing an upper anda lower conducting electrode associated with said insulator to provideone of said spark gaps, the relationship between said upper and lowerelectrodes being such as to cause sparkover therebetween to occur withinsaid insulator, means for venting said insulator, means for connectingsaid lower electrode to ground, and capacity means including said upperand lower electrodes disposed within said hollow insulator, a groundedconducting member, and means for supporting said conducting member atsuch a position in the electrostatic field of said structure as torestrict the entrance of fiux to said capacity means except through saidupper electrode.

5. In an excess voltage structure having a plurality oi spark gaps inseries circuit relation from conductor to ground, a hollow tubularinsulator having gas evolving wall material therein, an upper and alower conducting electrode associated with said insulator to form one ofsaid spark gaps, structural means for causing sparkcver between saidelectrodes to accur within said insulator to provide an enclosed sparkgap, means for venting said insulator, means for maintaining-said lowerelectrode at substantially ground potential and capacity meanselectrically connected in shunt relation with said enclosed spark gap,said capacity means including a grounded metal shield supported at sucha position in the electrostatic field as to prevent the concentration01' fiux between the electrodes of said enclosed gap.

6. In an excess-voltage protective structure subject to a voltage fromconductor to ground, insulating means supporting normally insulatedconducting electrodes in spaced relation to provide a first spark gapand a second spark gap in series relation from conductor to ground, ahollow insulator comprising insulating material within said insulatorsubject to thermal damage from creepase current, structural means forcausing sparkover 01' said second gap to occur within said insulator,and fiux control means including a conducting shield at ground potentialassociated with said structure for reducing both the time whereby damageto said insulating lag of sparkover of said structure and the pormalvoltage gradient across said second gap, material due to said creepagecurrent is prevented.

,7. An arc extinguishing device comprisinga first hollow insulator,upper and lower conductm ing electrodes at the respectiveends thereof,said elcctrodesbeing so arranged withrespect to said hollow insulator asto cause sparkover between said electrodes to occur withinsaid firsthollow insulator, a second hollow insulator supported within said firsthollow insulator, means ior increasing the capacity of the deviceincluding an. integral portion oi said lower electrode disposed withinsaid second hollow insulator to tr'ode and-said integral portion of saidlower electrodewith the, wall of said second hollow insulator.interposed therebetween, said integral portion of said, lower electrodehaving a relaprovide a condensercomprising said upper electively highresistance adapted to suppress voltage oscillations resulting irom adischarge between said electrodes.

8. An arc extinguishing device comprising a tubular insulator closed];at the upper end and open at the lower end, a cylindrical element 0!insulating material within said insulator, means supporting said elementoi insulating material in lateral abutting engagement with saidinsulator, a first conducting electrode carried by said element andextending downwardly from the closed end of said insulator and a secondconducting electrode positioned adjacent the open end of said insulator,said electrodes being normally insulated from each other and so relatedthat the shortest path therebetweenis along the line of abutment of saidelement of insulating material with the inner surface 01' said tubularinsulator.

9. An arc extinguishing device comprising a means holding said elementoi insulating material in lateral abutting relation with said tube, andconducting electrodes spaced apart to provide a preferential spark paththerebetween internally 01' said tube and normally insulated from eachother, said electrodes being so arranged that the weakest electricalpath therebetween is along the line of abutment of said element ofinsulating material with said hollow tube, at least one oi saidelectrodes being positioned in said annular space.

10. An arc extinguishing device comprising a tubular insulator closed atthe upper end and open at the lower end, an element of insulatingmaterial eccentrically supported in lateral abutting engagement with theinner surface of said insulator, a first conducting electrode carried 7by said element and extending downwardly from the closed end of saidinsulator and a second conducting electrode positioned in spacedrelationship with said first electrode, the arrangement oi saidelectrodes being such that the weakest electrical path therebetween isalong the line oi! abutment between said insulator and said insulatingelement, and resilient means opposing the movement of said element in alateral direction away from said weakest electrical path.

11. An arc extinguishing device comprising a tubular insulator closed atthe upper end and open at the lower end, an element of insulatabuttingengagement with the *inner' suria'ce of said insulator, a'firstconducting electrode carried by said elementand' extending "downwardlly'from thefclosedend of said-insulatorand'a "second conducting electrodepositionedins'pabe'd relationship with said first' electrode; the ar-'said' electrodes being such that rangement of r r eakest' electricalpath therebetween is along the the line of abutment between saidinsulator and said insulating'element, and resilient means arranged tofcontinuouslybias said element of insulating material toward weakestelectrical 4 path.

' 12; An are extinguishingdevice comprising a tubulaifinsulatorclosed atthe upper end and 7 open at the lower end, a cylindrical elementof vinsulating support n material within said insulator; means said-elementor insulating material H v. i 9 3 4 ing' material eccentricallysupported in lateral end-of said that the shortest path ther'ebetween isalong the line of' abutment'of said element of] insulatin material withthe inner surface of said tubular insulator, and means including anintegral portion ofon'of 'said electrodes 'disposed within saidcylindrical element of insulating material for increasing the electricalcapacity'ot the"de-- vice. 1 r v r "13. An arcextinguishing' devicecomprising a first hollow insulator, a second hollow'insulator containedby said first hollow insulator, normal- -ly insulated conductingelectrodes associated --'with said second hollow insulator,saidelectrodes beingsorelated that the weakest electrical paththerebetween 'isin aspajce between the inner surface or said firsthollow insulator and the outer surface or said second hollowinsulator;-' and v means including integral portions ofsaidial'ectrodes-disposed respectively on the inner and outer surfacesof said second insulator tor increasing. the' electrical capacity of the-device. a RALPH R. PrrrMAN.

r insulatonsaid'electrodes being nor- 'mally insulated fromeach'otlierand so-related

